Conductive paint having good adhesion to molding of metallic oxide

ABSTRACT

A conductive paint having a good adhesion to a molding of metallic oxide comprises: 100 parts by weight of silver-plated copper powder containing silver plating in the amount of not more than 30 wt %; 13.6-6.0 parts by weight of phenolic resin (solid content); 0.2-0.7 part by weight of one selected from saturated fatty acids, unsaturated fatty acids, metallic salts thereof, and coupling agents containing saturated fatty acids or unsaturated fatty acids; 1.0-4.0 parts by weight of triethanolamine; and 0.1-1.0 part by weight of dihydroxybenzene. The phenolic resin is a resol phenolic resin having an infrared spectrum that satisfies the following relationships: 
     (A) 1/n=0.8-1.2 
     (B) m/n=0.8-1.2 
     (C) b/a=0.8-1.2 
     (D) c/a=1.2-1.5 
     wherein 1, m, n, a, b and c are, respectively, infrared transmittances of 2-monosubstitution product, 2,4-disubstitution product, 2,4,6-trisubstitution product, methylol group, dimethylenether, and phenol group.

TECHNICAL FIELD

This invention relates to a conductive paint having a good adhesion to amolding of metallic oxide.

BACKGROUND ART

Moldings of metallic oxide, such as ITO (indium tin oxide) and IO(indium oxide), are widely used as transparent conductive films forelectrodes in liquid crystal displays and solar batteries. In order toattach a lead to such a molding of metallic oxide, silver paste (paintin a paste form containing silver powder as main component) is generallyused as a soldering electrode.

On the other hand, silver paste is not only expensive but usuallycontains epoxy resin as a binder in order to attain high adhesion to themolding of metallic oxide. Thus, good solderability and adhesion areachievable only if hardening conditions for the paste are controlledwith high precision.

An object of this invention is to provide a conductive paint whichsatisfies the following requirements:

(1) good adhesion to a molding of metallic oxide such as ITO and IO;

(2) good solderability, especially in reflow soldering; and

(3) satisfying the requirements (1) and (2) in a wide range of hardeningconditions.

DISCLOSURE OF THE INVENTION

In order to achieve the above object, according to the presentinvention, there is provided a conductive paint having a good adhesionto a molding of metallic oxide, the conductive paint comprising 100parts by weight of silver-plated copper powder containing silver platingin the amount of not more than 30 wt %; 13.6-6.0 parts by weight ofphenolic resin (solid content); 0.2-0.7 part by weight of one selectedfrom saturated fatty acids, unsaturated fatty acids, metallic saltsthereof and coupling agents containing saturated fatty acids orunsaturated fatty acids; 1.0-4.0 parts by weight of triethanolamine; and0.1-1.0 part by weight of dihydroxybenzene,

the phenolic resin being a resol phenolic resin which satisfies thefollowing relations:

(A) 1/n=0.8-1.2

(B) m/n=0.8-1.2

(C) b/a=0.8-1.2

(D) c/a=1.2-1.5

wherein 1, m, n, a, b and c are infrared ray transmittances of 2-monosubstitution product, 2, 4-di substitution product, 2, 4, 6- trisubstitution product, methylol group,

dimethylenether and phenol group, respectively.

The aforementioned conductive paint is made by adding an appropriateamount of antifoamer and a solvent comprising butylcarbitol andacetylacetone in the weight ratio of 9.5/0.5-8.0/2.0 and kneadedtogether.

The conductive paint according to the present invention satisfies all ofthe above-noted requirements.

If the amount of the silver plating in the silver-plated copper powderexceeds 30 wt %, silver in the paint will migrate into or be taken oreaten by the solder because silver quickly diffuses into the solderduring soldering.

If the content of the phenolic resin as a binder exceeds 13.6 wt %, thesolderability will not be good. On the other hand, if it is less than6.0 wt %, the silver-plated copper powder will not bind satisfactorilyand the adhesion will worsen. With the ratio of silver-plated copperpowder to binder, as the amount of the binder decreases, the adhesion tomolding of metallic oxide will increase for a moment. This is consideredto be due to the fact that the heat history during the heat hardeningprocess is released by the increase in the ratio of the silver-platedcopper powder. As for the transmittances of the phenolic resin definedin (A) to (D), see Unexamined Japanese Patent Publication No. 2-16172.

Saturated fatty acid may be palmitic acid, stearic acid or arachic acidhaving a carbon number of 16-20; unsaturated fatty acid may bepalmitoleic acid, oleic acid or linolenic acid having a carbon number of16-18; metallic salt thereof may be salts combined with metal such assodium, potassium, copper, aluminum or zinc. Coupling agents containingsaturated fatty acid or unsaturated fatty acid may be isopropyltristearoyl titanate, isopropyl trioctanol titanate or isopropyldimethacryl isostearoyl titanate.

These dispersants are added for better dispersion of the silver-platedcopper powder. But if it exceeds the upper limit, the adhesion of paintto ITO or IO will be poor. On the other hand, if it is below the lowerlimit, the dispersion will worsen, thereby lowering the solderability.

Triethanolamine is added to improve the adhesion of molding of metallicoxide to ITO or IO. Considering the solderability, 2.5 parts by weightof triethanolamine will be most appropriate. If its content exceeds theupper limit, the solderability will decrease. If it is below the lowerlimit, the adhesion to ITO or IO will be poor.

Dihydroxybenzene may be catechol, resorcinol or hydroquinone. It isadded to improve the adhesion to ITO or IO and to keep silver paste frombeing taken or eaten by solder during reflow soldering. If more than 1.0part by weight, the solderability will worsen. If less than 0.1 part byweight, the adhesion will decrease and it will become difficult toprevent the silver paste from being taken or eaten by the solder. Thus,its content should preferably be 0.1-1.0 part by weight.

Antifoamer may be acryl or silicon group and its content should bewithin 0.05-0.3 part by weight. If below the lower limit, foam willremain in the paste during screen-printing. If above the upper limit,the adhesion to ITO or IO will be lower.

As for solvent, the weight ratio of butylcarbitol to acetylacetoneshould preferably be between 9.5/0.5 and 8.0/2.0. The amount of solventshould be adjusted to keep the viscosity of the paste to such a level asto make screen-printing possible. If the acetylacetone ratio is belowthis range, the wettability with respect to ITO or IO will be poorduring printing. If above it, the solvent will volatilize so quickly asto worsen the printability.

BEST MODE FOR CARRYING OUT THE PRESENT INVENTION Example

First, materials of Articles 1-9 according to the present invention andControl Articles 10-17 having compositions in part by weight shown inTable 1 were prepared. Each of them was put in an appropriate containerand weighed. They were then kneaded with a roll mill for 20 minutes intopaste forms (conductive paints).

They were subjected to the following tests. The results are shown at thebottom of Table 1. In any of the tests, the hardening temperature waskept within the range of ± 20° C. of the standard hardening temperaturefor each paste.

Specific Resistance

Each paste was screen-printed with a 180-mesh Tetron screen on a glassepoxy substrate to an area 1 mm wide and 60 mm long. It was then heatedand hardened in an air oven at 160° C. for 30 minutes. The resistanceand thickness of the hardened paste 60 mm long were measured todetermine the specific resistance.

Solderability

Each paste was applied on ITO on stainless steel foil on which ITO hadbeen deposited, with a 180-mesh Tetron screen. It was heated andhardened in an air oven at 160° C. for 30 minutes. Solder cream wasscreen-printed on the thus hardened paste and it was subjected to reflowheating at 230° C. for 2 minutes. We checked the wettability of thesolder cream with respect to a paste coating.

Criterion for Evaluation

⊚: 100% of area of the paste coating was uniformly wetted with solder.

◯: Not less than 90% of the area was wetted with solder.

Δ: 30 to 90% of the area was wetted with solder.

X: Less than 30% of the area was wetted with solder.

Adhesion

Each paste was applied on ITO on stainless steel foil on which ITO hadbeen deposited, with a 180-mesh Tetron screen. It was heated andhardened in an air oven at 160° C. for 30 minutes. The thus hardenedpaste coating 10 mm × 10 mm was cross-cut by the width of 1 mm to dividethe coating into 100 sections. Then, cellophane tape was affixed to andpeeled off the coating to determine the adhesion of the coating to ITO.

Criterion for Evaluation

⊚: No peeling or chipping observed in any of the 100 square sections.

◯: No peeling but slight chipping observed in some of the 100 squaresections.

Δ: 1-99 square sections peeled off.

X: All of the sections peeled off.

Pulling Strength

Each paste was applied on ITO on stainless steel foil on which ITO hadbeen deposited, with a 180-mesh Tetron screen so that its area will be 3mm diameter. It was heated and hardened in an air oven at 160° C. for 30minutes. Solder cream was screen-printed on the thus hardened paste andsubjected to reflow heating process in an air oven at 230° C. for 2minutes. As a lead wire for the soldered paste, a 0.6-mm diametertin-plated annealed copper wire was soldered on the paste. We checkedhow firmly the paste was bonded to the ITO film by pulling the lead.

Criterion for Evaluation

◯: Bonded firmly. X: Peeled off easily.

Printability

Each paste was screen-printed 100 times with a 180-mesh Tetron screen ona glass epoxy substrate. We checked the printability.

Criterion for Evaluation

◯: No change in viscosity of the paste was observed during printing.

X: The viscosity changed during printing, making it difficult tocontinue printing.

The materials used in all Articles according to the present inventionand Control Articles are as follows.

Silver plated copper powder: containing 11 wt % of silver plating

Phenolic resin: made by Gunei Chemical Industry

(Trade name: Regitop PL4348)

Titanium coupling agent 1: isopropyl tristearoyl titanate

Titanium coupling agent 2: isopropyl trioctanol titanate

As is apparent from the above results, all of the Articles according tothe present invention attained good evaluation (which means that theycan stand practical use in all the tests). However, the Control Articleshad bad evaluation (which means that they are practically useless) insome of the tests. It is confirmed that all of the Articles according tothe present invention are satisfactorily useful as conductive paints formoldings of metallic oxide.

Next, we compared silver plating amount (parts by weight) insilver-plated copper powder by using Articles 1 and 2 and ControlArticles 3 and 4 shown in Table 2. First, each of them was put in anappropriate container and weighed. They were kneaded into paste with aroll mill for 20 minutes. We conducted for these pastes the same testsas the above-described tests (Specific Resistance), (Solderability),(Adhesion) and (Pulling Strength) under the same conditions and also atest on how much the paste is eaten by the solder. The results are shownin Table 2. In the last-mentioned test, we visually checked whether thepaste was taken or eaten by the solder during the solderability testafter the reflow heating process. ◯: shows that it was not taken oreaten in this test, whereas shows that it was eaten.

In all of the above tests, all of the Articles according to the presentinvention attained good evaluation (practically usable). However, theControl Articles had bad evaluation (practically useless) in tests forpulling strength and in the last-mentioned test. It is confirmed thatthe Articles according to the present invention are satisfactorilyuseful as conductive paints for moldings of metallic oxide.

Industrial Application

According to the present invention, the present invention can eliminatethe necessity to control the hardening condition with high precision inthe silver paste containing an epoxy resin binder, for good adhesion andsolderability. Also, even if the hardening temperature varies within therange of ± 20° C. of the standard hardening temperature, it is possibleto obtain good conductivity, adhesion and solderability. Thus,productivity will be improved markedly.

                                      TABLE 1                                     __________________________________________________________________________                  Article according to the present invention                                    1   2   3   4   5   6   7   8   9                               __________________________________________________________________________    Silver-plated copper powder                                                                 100 100 100 100 100 100 100 100 100                             Phenolic resin                                                                              13.5                                                                              6.5 11.1                                                                              11.1                                                                              11.1                                                                              11.1                                                                              11.1                                                                              11.1                                                                              11.1                            Titanium coupling agent 1                                                                   0.5 0.5     0.7             0.5 0.5                             Titanium coupling agent 2                                                                           0.3     0.5     0.5                                     Oleinic acid potassium            0.5                                         Triethanoleamine                                                                            2.5 2.5 2.5 2.5 1.5 3.5 2.5 2.5 2.5                             Catechol      0.25                                                                              0.25        0.25                                                                              0.25                                        Hydroquinone          0.25                                                                              0.25        0.15                                                                              0.90                                                                              0.25                            Antifoamer    0.06                                                                              0.06                                                                              0.06                                                                              0.06                                                                              0.06                                                                              0.06                                                                              0.06                                                                              0.06                                                                              0.06                            Solvent       9/1 9/1 9/1 9/1 9/1 9/1 9/1 9/1 8/2                             butylcarbitol/acetylacetone                                                   Specific resistance                                                                         2.0 1.3 2.2 1.8 1.5 1.7 1.9 1.5 2.0                             (× 10.sup.-4 Ω cm)                                                Solderability ◯                                                                     ⊚                                                                  ⊚                                                                  ⊚                                                                  ⊚                                                                  ⊚                                                                  ⊚                                                                  ◯                                                                     ⊚                Adhesion      ◯                                                                     ⊚                                                                  ⊚                                                                  ◯                                                                     ◯                                                                     ⊚                                                                  ◯                                                                     ⊚                                                                  ⊚                Pulling strength                                                                            ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                   Printability  ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                   __________________________________________________________________________                      Control article                                                               10  11  12  13  14  15  16  17                              __________________________________________________________________________    Silver-plated copper powder                                                                     100 100 100 100 100 100 100 100                             Phenolic resin    14.0                                                                              5.0 11.1                                                                              11.1                                                                              11.1                                                                              11.1                                                                              11.1                                                                              11.1                            Titanium coupling agent 1                                                                       0.5         0.8 0.5 0.5                                     Titanium coupling agent 2                                                                           0.5 0.1             0.5                                 Oleinic acid potassium                        0.5                             Triethanoleamine  2.5 2.5 2.5 2.5 5.0 0.5 2.5 2.5                             Catechol          0.25                                                                              0.25        0.25                                                                              0.25                                    Hydroquinone              0.25                                                                              0.25        2.0 0.05                            Antifoamer        0.06                                                                              0.06                                                                              0.06                                                                              0.06                                                                              0.06                                                                              0.06                                                                              0.06                                                                              0.06                            Solvent           9/1 9/1 9/1 9/1 7/3 9/1 9/1 7/3                             butylcarbitol/acetylacetone                                                   Specific resistance                                                                             2.2 1.8 1.6 1.5 1.3 1.8 1.4 1.5                             (× 10.sup.-4 Ω cm)                                                Solderability     Δ                                                                           Δ                                                                           X   ⊚                                                                  X   ⊚                                                                  X   ⊚                Adhesion          X   ◯                                                                     ⊚                                                                  X   ⊚                                                                  X   ⊚                                                                  X                               Pulling strength  X   X   X   X   X   X   X   X                               Printability      ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                   __________________________________________________________________________

                  TABLE 2                                                         ______________________________________                                                      Article                                                                       according                                                                     to the                                                                        present       Control                                                         invention     article                                                         1    2        3      4                                          ______________________________________                                        11% (silver plating)                                                                          100                                                           25% (silver plating)   100                                                    2% (silver plating)             100                                           35% (silver plating)                 100                                                      The compositions of the articles are                                          the same as Article 9 in Table 1                                              except for the silver-plated copper                                           powder.                                                       Specific resistance                                                                           2.0    1.6      2.2  1.4                                      (× 10.sup.-4 Ωcm)                                                 Solderability   ⊚                                                                     ⊚                                                                       ⊚                                                                   ◯                            Adhesion        ⊚                                                                     ⊚                                                                       ⊚                                                                   ⊚                         Pulling Strength                                                                              ◯                                                                        ◯                                                                          X    X                                        Paste eaten condition                                                                         ◯                                                                        ◯                                                                          ◯                                                                      X                                        ______________________________________                                    

What is claimed is:
 1. A conductive paint having a good adhesion to amolding of metallic oxide, said conductive paint comprising 100 parts byweight of silver-plated copper powder containing silver plating in theamount of not more than 30 wt %; 13.6-6.0 parts by weight of phenolicresin (solid content); 0.2-0.7 part by weight of one selected fromsaturated fatty acids, unsaturated fatty acids, metallic salts thereof,and coupling agents containing saturated fatty acids or unsaturatedfatty acids; 1.0-4.0 parts by weight of triethanolamine; and 0.1-1.0part by weight of dihydroxybenzene,said phenolic resin being a resolphenolic resin which satisfies the following relations:(A) 1/n=0.8-1.2(B) m/n=0.8-1.2 (C) b/a=0.8-1.2 (D) c/a=1.2-1.5 wherein 1, m, n, a, band c are infrared ray transmittances of 2-mono substitution product, 2,4-di substitution product, 2, 4, 6-tri substitution product, methylolgroup, dimethylenether and phenol group, respectively.
 2. A conductivepaint having a good adhesion to a molding of metallic oxide, wherein aneffective amount of antifoamer and a solvent comprising butylcarbitoland acetylacetone in the weight ratio of 9.5/0.5-8.0/2.0 is added to andkneaded with said conductive paint claimed in claim 1.